(1) Field of the Invention
The present invention relates to polymer particles. More particularly, the present invention relates to a process for the preparation of polymer particles in which the molecular weight of the interior is different from the molecular weight of the surface layer portion and which are used as extrusion molding resins, optical molding resins and other excellent products, and particles prepared according to this process.
Furthermore, the present invention relates to a process for the preparation of toner particles to be used for the electrophotography or electrostatic printing, and a toner comprising particles prepared according to this process. More particularly, the present invention relates to a toner formed by incorporating a colorant and the like into the above-mentioned polymer particles, which has a sharp particle size distribution, good flowability and blocking resistance, and excellent developing and fixing properties.
(2) Description of the Related Art
In polymer particles, the weight average molecular weight is substantially the same at any parts of particles. Namely, the polymer constituting the surface layer of particles has substantially the same average molecular weight as that of the polymer constituting the interior of particles, and the properties are not substantially different throughout the particles. However, particles having a sufficient adhesiveness on the surface but having an interior portion which does not melt, such as foamed styrol particles, and products having a high refractive index in the interior and a low refractive index in the surface portion, such as spherical lenses, are now required for high techniques. In case of low-molecular-weight particles such as extrusion molding resins, the surface has a high adhesiveness and particles are readily bonded to one another at the extrusion molding, and therefore, development of particles having no adhesiveness on the surface is desired.
However, most of conventional resins composed of polymer particles fail to sufficiently satisfy these requirements. Accordingly, with a view to providing functionally excellent polymer particles, we improved the suspension polymerization process in order to prepare polymer particles having different molecular weights in the interior portion and the surface layer portion.
In the suspension polymerization process, since the polymerization of a monomer is carried out in oil drops suspended and dispersed in an aqueous medium, the polymerization heat can be easily removed. By dint of this advantage, the suspension polymerization process is widely utilized on an industrial scale.
In this suspension polymerization process, however in order to stably suspend and disperse a monomer and the like into an aqueous medium, it is necessary to use surface active agents or various hardly soluble inorganic salts.
Accordingly, these surface active agents and the like are readily incorporated as impurities in the formed polymer and have bad influences on characteristics, such as electric properties, of the formed polymer.
The formed polymer is obtained in the particulate shape in the suspension polymerization process, and if it is intended to obtain particles having a small particle size, for example, a particle size smaller than 10 .mu.m, it is necessary to increase the amount used of a dispersant such as a surface active agent, and therefore, emulsion polymerization particles having a particle size smaller than 1 .mu.m are formed and they are incorporated as impurities in the formed polymer.
In the field of the electrophotography, a toner is used for visualizing an electrostatic image. Particles of this toner are composed of a composition formed by incorporating a colorant and, if necessary, a charge controlling agent and other additives in a resin medium, and the particle size is adjusted within a certain range, for example, from 1 to 30.mu.m. A resin having desirable electroscopic and binding properties, for example, a styrene resin, is used as the resin medium, and carbon black or an organic or inorganic coloring pigment is used as the colorant.
As the process for the preparation of such a toner, there is known a process in which a toner is directly prepared at the polymerization step for preparing a toner resin.
In general, this preparation process comprises dissolving a water-insoluble monomer in a polymerization initiator soluble in the monomer, adding a colorant and other additive to the solution, suspending the resulting composition into an aqueous solution comprising a dispersant such as a water-soluble polymer, an inorganic powder and a surface active agent under high-speed shearing agitation and effecting polymerization to prepare colored polymer particles.
In this suspension polymerization process, the particle size of the final toner is determined by the suspension state of the monomer composition in water, and the particle size distribution is much broadened according to the state of charging of the monomer composition into water and the stirring state, and it is very difficult to obtain particles having a uniform particle size. Moreover, coarse particles having a particle size of scores of .mu.m to several mm are generally formed in this suspension polymerization process, and it is difficult to obtain particles having a particle size of 1 to 30 .mu.m valuable for a toner of a developer. Of course, it is possible to reduce the particle size of the formed polymer by increasing the amount incorporated of the dispersant. In this case, however, the dispersant is included in the toner, and the toner becomes sensitive to the moisture and the electrophotographic characteristics are readily degraded. A particular post treatment is necessary for eliminating this disadvantage, and the number of steps increases and the process is not practically applicable.
The most serious problem of the toner prepared by the suspension polymerization process resides in the unevenness of the particle size. Namely, incorporation of fine particles at a specific ratio cannot be avoided. These fine particles are composed solely of a polymer not containing toner additives such as the colorant and have characteristics different from those of the toner particles. Accordingly, even if these fine particles are incorporated in small amounts, scattering of the toner or fogging is caused at the development step and there is a risk of reduction of the flowability of the toner. Even if these fine particles can be removed, the composition of the obtained toner particles is greatly different from the initial composition, and the desirable characteristics cannot be obtained and the cost is increased by reduction of the yield.
Various proposals have been made on the process for preparing a toner having improved developing and fixing properties by the suspension polymerization. For example, Japanese Examined Patent Publication No. 64-11941 discloses a process in which a plurality of initiators differing in the half-value period are contained in a polymerizable monomer and the polymerization is carried out in this state. In this process, however, if an initiator having a long half-value period is used to broaden the molecular weight distribution, the time required for the polymerization is long, and the period during which oil drop particles are dispersed in the highly viscous state at the polymerization becomes long and coalescence of particles is readily caused, with the result that the formed toner contains coarse particles and the particle size distribution is broadened. The so-obtained toner shows certain effects in connection with the fixing property and offset resistance, but the durability and blocking resistance are poor, presumably because of the presence of the low-molecular-weight component on the surfaces of the formed polymer particles. Accordingly, further improvements are desired.